A transformer has become an essential electronic component for voltage regulation into required voltages for various kinds of electric appliances.
Since the leakage inductance of the transformer has an influence on the electric conversion efficiency of a power converter, it is very important to control leakage inductance. In the power supply system of the new-generation electric products such as LCD televisions, leakage inductance transformers (e.g. LLC transformers) become more and more prevailing. Generally, the current generated from the power supply system will pass through a LC resonant circuit composed of an inductor L and a capacitor C, wherein the inductor L is inherent in the primary winding coil of the transformer. At the same time, the current with a near half-sine waveform will pass through a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) switch. When the current is zero, the power MOSFET switch is conducted. After a half-sine wave is past and the current returns zero, the switch is shut off. As known, this soft switch of the resonant circuit may reduce damage possibility of the switch, minimize noise and enhance performance. As the LCD panels become more and more large-sized and slim, many components (e.g. magnetic elements, conductive winding modules, or the like) are developed toward minimization and high electric conversion efficiency.
FIG. 1 is a schematic exploded view of a conventional leakage inductance transformer. As shown in FIG. 1, the transformer 1 comprises a bobbin 11, a covering member 12, and a magnetic core assembly 13. A primary winding coil 111 and a secondary winding coil 112 are wound around the bobbin 11. The output terminals 113, 114 of the primary and the secondary winding coils 111, 112 are directly wound and soldered on pins 115, which are perpendicularly extended from the bottom of the bobbin 11. The cover member 12 is used for partially sheltering the upper portion of the bobbin 11 in order to increase the creepage distances between the primary winding coil 111, the secondary winding coil 112 and the magnetic core assembly 13. The magnetic core assembly 13 includes middle portions 131 and leg portions 132. The middle portions 131 are accommodated within a channel 116 of the bobbin 11. The bobbin 11 is partially enclosed by the leg portions 132. Meanwhile, the transformer 1 is assembled.
As known, after the transformer 1 is assembled, an air gap (not shown) is defined between the corresponding leg portions 132. The air gap is formed between the primary winding coil 111 and a secondary winding coil 112. If the secondary winding coil 112 is in a short-circuit condition, the magnetic path possibly causes individual loops and thus the leakage inductance is increased. Under this circumstance, the leakage inductance of the transformer 1 fails to be stably controlled. In addition, after the outlet parts 113 and 114 of the primary winding coil 111 and the secondary winding coil 112 are wound around and soldered on the pins 115, each of the outlet parts 113 and 114 is usually sheathed by a tube 14. If the tube 14 is omitted, the primary winding coil 111 and the secondary winding coil 112 wound around the bobbin 11 are possibly stained with solder paste because the wire-managing groove 117 is too short or the distance between the pin 115 and the winding section of the bobbin 11 is too short. Although the use of the tube 14 could protect the primary winding coil 111 and the secondary winding coil 112 wound around the bobbin 11, there are still some drawbacks. For example, the tube 14 may be thermally damaged. The procedure of sheathing the tube 14 is time-consuming and labor-intensive. In addition, the use of the tube 14 increases the cost of the transformer.
Therefore, there is a need of providing an improved transformer so as to obviate the drawbacks encountered from the prior art.